1. Field of the Invention
The present invention relates to a sensor device, in which a detection circuit is electrostatically shielded.
2. Description of the Prior Art
In a proximity sensor of an induction type, there has been known that an insulating flexible film having a conductor pattern is wound around a printed wiring board having a circuit unit mounted thereon, so that the printed wiring board is electrostatically shielded, and further, that the conductor pattern is soldered to a deposition film formed at the back surface of a core (see Patent Literature 1 which is Japanese Patent Application Laid-open No. 1,436/1981).
Furthermore, there has been known that in a coil device, in which a coil wire is applied to a magnetic core, a fine wire of a metallic magnet such as permalloy or silicon steel is wound around the coil wire, so that the coil wire is magnetically shielded (see Patent Literature 2 which is Japanese Patent Application Laid-open No. 195,542/1999).
The proximity switch which adopts the electrostatic shield structure disclosed in Patent Literature 1 has raised problems that: a film with a copper foil, which is subjected to secondary machining in a complicated shape and is expensive in material cost, is needed in each kind of sensor; it is cumbersome to manually wind the film with a copper foil having a complicated shape around the circuit board, thereby increasing the man-hours of assembly; it is difficult to automatically wind the film with a copper foil having a complicated shape around a circuit board; the man-hours of soldering a copper foil portion of the film with a copper foil to a GND pattern of the circuit board is large; it is difficult to automatically solder the copper foil portion of the film with a copper foil to the GND pattern of the circuit board; a film with a complicated electrostatic shield copper foil is needed by disposing a lead wire or a welded portion for electrically welding the metallic deposition portion of the core to the GND pattern of the circuit board; it is difficult to automatically solder the metallic deposition portion of the core and the GND pattern of the circuit board to each other by the use of the lead wire or the film with a copper foil; it is necessary to apply an insulating layer to the surface of the film with a copper foil in order to secure an insulating property between electronic parts or a sheath case, thereby requiring the film with a copper foil excellent in material cost; the surface of the insulating layer is peeled off when the insulating layer scrapes on the edge of the circuit board or the like since the insulating layer is relatively thin, thereby inducing short-circuiting between the electronic parts or circuit patterns; and the like.
The shield structure disclosed in Patent Literature 2 has not been suitable for electrostatically shielding a detection circuit in a sensor device since a member to be shielded is not a circuit board but a coil and a magnetic material wire is thickly wound for the purpose of magnetic shielding.